The present invention relates to a method for identifying base stations of a TDMA cellular network in a mobile station, and also to a mobile station for use in such a method.
As regards base station identification the GSM cellular network in its present state operates as follows. A neighbour list is defined for each cell in the GSM network, containing the frequencies of the broadcast control channels (BCCH) of the base stations to which the mobile station can move on next. The mobile station measures field strengths at frequencies determined by the base station broadcast control channel BCCH, averages each of them and, based on the results, compiles a list of six strongest neighbour base stations. An attempt is made to decode the identification codes of the six strongest neighbours and the levels and identification codes of those base stations the identification codes of which have been decoded are reported to the network. The network uses the information to make a handover decision.
In a cellular network, the frequencies are re-used, i.e. each particular frequency is used simultaneously by several cells. Therefore, when measuring field strengths a mobile station periodically reads the base station identification code (BSIC) transmitted on the synchronisation channel (SCH) by the base station measured. Base stations using the same broadcast control channel BCCH frequency have different BSICs. When the BSIC is found, it tells the mobile station whether the base station in question is a neighbour base station or some other base station transmitting from farther away. This is why the BSICs are read. If a BSIC cannot be re-confirmed within a certain time period or if an incorrect BSIC is detected, the current field strength measurement related to the frequency in question will be discarded.
One time period, so-called IDLE frame, which is a little longer than a time division multiple access (TDMA) frame, is reserved in the mobile station""s traffic channel (TCH) multiframe for the reading of BSICs. During that frame, one TDMA frame can be read from data coming from a base station. The BSIC appears in the base station""s BCCH/CCCH multiframes every 10 or 11 TDMA frames (CCCH=common control channel). The BCCH/CCCH multiframe consists of 51 TDMA frames. The IDLE frame, in turn, is repeated every 26 TDMA frames. Thus, the IDLE frame xe2x80x9cslipsxe2x80x9d on the BCCH/CCCH multiframe for one TDMA frame per BCCH/CCCH multiframe cycle. When looking for a BSIC related to a certain BCCH frequency, it is first read the FCCH burst in the frequency correction channel (FCCH) frame of the BCCH/CCCH multiframe and then the BSIC in the following SCH frame. In the best case, the FCCH frame coincides with the IDLE frame immediately and in the worst case, only after 11 IDLE frames. So, the BSIC is detected during 2 to 13 TCH multiframes, on average after about 7 multiframes, i.e. in 0.8 seconds, approximately.
The problem with the operation described above is the relatively long time it takes to identify new base stations and in particular to find the BSICs. Therefore, especially a fast-moving mobile station may fail to monitor the neighbour base stations in a small-cell network, in other words, the handover may sometimes fail. New base stations (frequencies) appear on the neighbour list of a mobile station always after a handover, and even when moving within a single cell the list of the six strongest neighbours may be modified quite often. Furthermore, the traffic channel of a cell is often heard on the list of the six strongest neighbours and since the mobile station does not know whether a signal is from a traffic channel or a neighbour from the BCCH multiframe, the IDLE frames used for the search for the FCCH frame at this particular frequency are wasted. The slowness of identification of new base stations is caused by the read procedure used and by the fact that the base stations are not synchronised with each other.
The mobile station tries to recognise the identification codes of the six strongest base stations as quickly as possible and at the same time the identification codes at the frequencies of already-identified base stations have to be regularly re-confirmed. Re-checks are necessary as otherwise noise frequencies may cause errors in the field strength figures. The reading of an old BSIC does not take much time as such because the mobile station knows the IDLE frame during which an already-detected BSIC can be re-read. The problem is that the greater part of the time, i.e. IDLE frames available, is used searching for the BSICs of new base stations.
An object of the invention is to provide a new method for identifying base stations of a time division cellular network in a mobile station and to eliminate the disadvantages described above.
In a first method according to the invention for identifying base stations of a time division cellular network, data are transferred from the serving base station to the mobile station concerning the frame timing differences of the neighbour base stations, in the mobile station it is determined on the basis of said frame timing differences the system data reception times for the neighbour base stations, and system data are read and neighbour base stations are identified.
In the first method according to the invention, the serving cell, or base station, transmits the frame timing differences of the base stations on the neighbour list to the mobile station. The frame timing differences have been calculated e.g. for a pseudo-synchronous handover. That calculation may be based on mutual base station timing differences measured and reported by the mobile phone. The frame timing difference data are transmitted in the active state preferably on a suitable control channel such as the slow associated control channel SACCH or the fast associated control channel FACCH, and, correspondingly, these data are transmitted in the idle state preferably on the broadcast control channel BCCH or the common control channel CCCH. The frame timing difference data can also be transmitted in some other suitable manner. The mobile station receives the frame timing difference data and uses them to calculate the reception times for the base stations"" system data. This means preferably determining the time of occurrence of the IDLE frames that are aligned with the BSICs transmitted by each of the base stations on the neighbour list. These IDLE frames are then used for reading the BSICs. So, here the mobile station does not search for the frequency correction channel FCCH, which search consumes the greatest amount of IDLE frame capacity.
Thus the invention has the advantage that the system data, preferably the identification codes, of the base stations corresponding to the new frequencies on the neighbour list can be read relatively fast.
Additionally the invention has the advantage that more time remains for checking the identification codes of the old base stations on the list since no IDLE frames are wasted searching for the timing/synchronisation data, i.e. FCCH frames, of the new base stations.
Furthermore, the invention has the advantage that when a mobile station is in the idle state, its energy consumption decreases from what it is presently. This is based on the fact that when the frame timing differences are known, less time is consumed monitoring the neighbour cells"" system data and the active time of the mobile station thus becomes shorter.
In a second method according to the invention for identifying base stations of a time division cellular network, data are transferred from the serving base station to the mobile station concerning the broadcast control channel time slots used by the neighbour base stations, in mobile stations it is determined on the basis of said broadcast control channel data the system data reception times for the neighbour base stations, and system data are read and a neighbour base station is identified.
The second method according to the invention is applied in a cellular network that uses discontinuous transmission of broadcast control channel BCCH carrier wave. The basic idea in the method is that the mobile station should know the time slots during which the transmission of the broadcast control channel BCCH of a particular neighbour cell is carried out and during which the neighbour cell monitoring and measurements can be performed.
In the present GSM system, each base station""s broadcast control channel BCCH carrier wave is continuously transmitted. This facilitates mobile station synchronisation with network as well as field strength measurements of neighbour base stations. On the other hand, a continuously transmitted broadcast control channel BCCH has the disadvantage that traffic channels on this carrier cannot employ discontinuous transmission DTX and power control. Furthermore, it should be noted that even if there were no traffic at all on the broadcast control channel BCCH, a so-called dummy burst must be transmitted all the same. This results in a decrease of capacity as the noise level in the network becomes higher. Bigger than in the GSM system the problem will be in future mobile telephone systems such as the universal mobile telecommunication system UMTS, in which the number of traffic channels on the broadcast control channel BCCH carrier can be considerably larger than in the GSM system (e.g. 64).
In a cellular network employing discontinuous broadcast control channel BCCH carrier, the search for the serving cell and the reliability of monitoring the neighbour base stations will be degraded. This is because carrier discontinuity results in considerable errors in the field strength measurements as a measurement may be carried out at a time when the carrier is not transmitted. One way of alleviating the problem is to lengthen the averaging period of the measurement. This, however, leads to the slowing-down of the search for the serving cell, i.e. presynchronisation, and the monitoring of neighbour base stations, especially when there is only a little traffic on the carrier. However, the problems described above can be solved using the second method according to the invention.
When a mobile station receives from the serving base station on the broadcast control channel BCCH the data about the neighbour base stations as well as the information about the time slots during which a neighbour base station""s BCCH carrier wave transmission is active, the mobile station can perform the monitoring and measurement of that neighbour base station. In a preferred embodiment of the invention, the data transmitted from the serving base station include the information about the frame timing differences of the neighbour base stations.
In a preferred embodiment of the invention the reading of system data and the identification of neighbour base stations include the field strength measurements. When the field strength measurement has to be performed asynchronously, i.e. without knowledge of whether the measured time slot is at reference power, it is advantageous according to the invention to improve the reliability of the measurement by having on the carrier a minimum number of time slots at a fixed transmission power, or reference power. The more there are such time slots, the more reliable the measuring information.
In the monitoring of neighbour base stations, the reliability of the field strength measurement (RSSI) is improved if the locations of the carrier reference power time slots in the broadcast control channel BCCH carrier are known. It is advantageous that the reference power time slot locations and frame timing differences with respect to other neighbour base stations are transmitted via the broadcast control channel BCCH of the serving base station. Additionally, if the locations of the broadcast control channel time slots used by the neighbour base stations are hopping because of time or frequency hopping, for example, information will be transmitted also on the hopping sequences of the other base stations.